Microwave ovens, now a permanent fixture in many homes, also increasingly find use in high volume industrial applications. For example, the tempering of large quantities of frozen meat, fish, poultry, and fruit is greatly enhanced with the use of microwave ovens. Not only do they provide for greater uniformity in tempering, they also eliminate several hour wait times to thaw a frozen product prior to its availability for use while minimizing drip loss and improving sanitation.
Just as with units designed for domestic use, industrial microwave ovens must be appropriately designed to prevent leakage of microwave energy from the cavity during operation. Compliance with government safety regulations is increasingly difficult with such units as general concerns about high doses of radiation increases. Ii is not uncommon for an industrial microwave oven, which may, for example be required to process several hundred kilograms of product in a several minute time span, to generate radio frequency energy levels of 50 kilo Watts (kW) or more. In addition, the proliferation of various types of wireless consumer devices such as cellular telephones which operate using milliwatt (mW) radio frequency power levels at microwave frequencies have prompted government agencies in several countries to further limit the acceptable amounts of radiation emanating from an oven.
Many different techniques have been developed to limit the amount of energy which is spuriously emitted by a microwave oven. Perhaps the most popular way is to seal the door of the oven with some type of radio frequency choke structure. Most such seals use the so-called one quarter wavelength blocking principle whereby a conductive choke structure is placed around the periphery of the door adjacent where the door meets the cavity when closed. The choke is designed such that conductive material is gapped at predetermined intervals equal to one quarter of the wavelength of the energy to be blocked. Simply placing a quarter wavelength choke within the door, being relatively inexpensive and straightforward to manufacture, is quite adequate for the domestic oven market.
However, the operating environments for commercial microwave ovens are far more stringent and thus require that additional measures be taken into consideration. Commercial ovens must typically be provided with a door that uses minimal floor space when opened, while at the same time providing maximum access for cleaning. And the spacing between door components and the oven cavity must be maintained over distances as large as a meter or more across. In addition, the effectiveness of the choke must not degrade over many hundreds of rapid open and close cycles. Indeed, in the past some have found it necessary to design oven doors which have mechanisms for adjusting the spacing between the door and the cavity in the field.